1. Field of the Invention
The present invention is related to a motor driven power steering apparatus which assists the steering mechanism of a vehicle by the turning force of a motor, and particularly to a motor driven power steering apparatus which improves reliability when the apparatus is in trouble.
2. Description of Related Art
Conventionally, a motor driven power steering apparatus of this kind controls the assist torque to reduce corresponding to steering load which becomes lighter as the vehicle speed increases so that the steering torque at a high speed running is not too light. Generally, gradient of the characteristic of relationship between the steering torque and motor current is so controlled as to decrease corresponding to the increase of the vehicle speed.
As a concrete method of controlling, in digital controlling which employs a microcomputer, such method that a data of characteristic of relationship between steering torque and motor current which has been previously stored in an inner memory unit as a table is read out corresponding to a vehicle speed signal inputted to the microcomputer to be controlled is employed.
On the other hand, in an analogue controlling, such method that motor current is controlled to be a predetermined value corresponding to an output signal and a vehicle speed signal of the steering torque sensor by using an operation amplifier, comparator, and various kinds of semiconductors is employed.
In either case of above digital controlling using the microcomputer or analog controlling, however, there was a big problem from a viewpoint of reliability because, when the microcomputer, operation amplifier, comparator, and semiconductors, etc. are in trouble by some reason, large current flows during high-speed running, leading to troubles that a steering wheel turns round uncontrollably in the direction against the driver's will or oversteers due to too much assist torque.